The present invention relates generally to a high power factor converter system, and more specifically to a method and apparatus for generating an output with high power factor input.
Many devices such as X-ray machines, ultrasound power supplies, and electronically commuted motors (ECMs) operate on power drawn from external alternating current power transmission lines. Converter systems are usually designed to draw power from the power transmission lines and to provide the power to such devices. Usually such converter systems are not purely resistive or linear. Therefore, the current and the voltage components of power lag or lead each other, and may substantially deviate from desired sinusoidal waves.
Power factor is a function of the root mean square value of current and voltage components of power. Mathematically, power factor is defined as the cosine of the phase angle between the voltage component and current component of power. Nonlinear loads such as rectifier circuits draw non-sinusoidal current from the converter system as well. Since such currents generally do not produce any useful power, they also contribute to the lowering of power factor. It is often desirable to maintain a high power factor so as to draw maximum power per current unit from the alternating current power transmission line. A low power factor results in undesirable transmission losses and low system performance.
Conventional converter circuits designed to assist in maintaining a high power factor typically include input and output stages. The input stage and the output stage of a converter circuit are usually connected using high power switches. High power switches are typically expensive and therefore add to the cost of the converter circuit.
In addition, converter circuits generally include relatively large electrolytic capacitors which are required to be charged periodically. The charging circuits of such converter circuits are usually charged using additional charging devices such as contactors, charging resistors, or combinations thereof. Such additional charging devices also add to the cost of converter circuits.
Therefore, what is desired is a method and apparatus for generating output power that results in a high power factor while minimizing the cost of implementation.
Briefly, in one embodiment of the invention, a high power factor converter comprises an input rectifier, a converter and a booster circuit. The input rectifier is configured for rectifying an input ac voltage. The converter is configured for generating an output voltage for the high power factor converter system, and comprises converter switches and inverse parallel diodes. The booster circuit comprises an inductor and at least one converter switch and at least one inverse parallel diode of the converter. The inductor is coupled between the input rectifier and the converter. The booster circuit is configured for controlling the supply of a booster current through the inductor to be discontinuous when an instantaneous voltage level of the input ac voltage is less than a reference voltage level and continuous when the instantaneous voltage level is greater than the reference voltage level.
Another aspect of the invention is a method for generating an output voltage with a high power factor input. The method comprises rectifying an input ac voltage, and using the rectified input ac voltage to obtain a booster current. The supply of a booster current is controlled to be discontinuous when an instantaneous voltage level of the input ac voltage is less than a reference voltage level and continuous when the instantaneous voltage level is greater than the reference voltage level. The controlled supply of the booster current is used for generating the output voltage.
Another embodiment of the invention provides a high voltage generator for driving an X-ray tube comprising an input rectifier, a converter and a booster circuit. The input rectifier is configured for rectifying an input ac voltage. The converter is configured for supplying an output voltage for the X-ray tube and comprises converter switches and inverse parallel diodes. The booster circuit comprises an inductor and at least one converter switch and at least one inverse parallel diode of the converter. The inductor is coupled between the input rectifier and the converter. The booster circuit is configured for controlling the supply of a booster current through the inductor, the current being discontinuous when an instantaneous voltage level of the input ac voltage is less than a reference voltage level and continuous when the instantaneous voltage level is greater than the reference voltage level.
Another embodiment of the invention provides a system generating an output voltage comprising means for rectifying an input ac voltage and means for using the rectified input ac voltage to obtain a booster current. The system comprises means for controlling the supply of the booster current to be discontinuous when an instantaneous voltage level of the input ac voltage is less than a reference voltage level and continuous when the instantaneous voltage level is greater than the reference voltage level. The system comprises means for generating the output voltage while maintaining high power factor input by using the controlled supply of the booster current.